The broad long-term objective of this work is to develop improved methods and strategies for achieving tumor specific delivery of drug and gene therapeutics. More specifically, the central aim of this project is to develop an efficient method for isolating peptides that exhibit specific molecular recognition and internalization into tumor cells. To identify the peptides most suitable for applications in gene delivery, methods will be developed to select peptide sequences that target tumor cells when genetically inserted into a viral gene therapy vector. Thus, a goal of this work is to couple efficient and highly effective peptide library selection and screening technologies to the development of targeted viral vectors. The selection for internalizing peptides will be accomplished using a two-step process. First, a very large bacterial display library of more than 50 billion unique sequences will be utilized for the first time to isolate peptides that mediate i) highly specific binding and entry into breast cancer tumor cells in culture and ii) minimal binding to peripheral blood cells and healthy tissue epithelial cells. This step will serve to process the largest possible diversity, and to ensure maximum tumor vs. non-tumor specificity, potentially utilizing low abundance tumor cell receptors. Tumor cell specificity will be favored by performing depletion selection steps, prior to each round of selection, upon on-transformed epithelial and fibroblast cell lines. Second, the diverse collection sequences resulting from the first step will be selected for their ability to mediate improved tumor targeting and transduction of a common viral gene therapy vector after incorporation into the viral capsid protein. Thus, this second selection step will provide a means to select for sequences, which are compatible with viral vector production, systemic delivery in the blood stream, and efficient transduction of the target tumor cells. Peptide sequences will be characterized for their specificity and apparent binding affinity and internalization rates by performing kinetic experiments with soluble peptides. The results and reagents obtained from this project should be immediately valuable for cancer diagnostics, and may provide effective new viral vectors for gene therapy. More generally, this work is anticipated to lead to new strategies for developing more specific and efficacious drug and gene delivery methods for cancer therapy.